It is known to incorporate additives into articles made from olefin polymers to prevent fading and embrittlement over time due to exposure to UV radiation. For example, European Patent Publication No. 343,717 discloses the use of polysiloxanes containing sterically hindered piperidino groups to stabilize polyolefins against ultraviolet radiation and heat. There is no disclosure concerning stabilization against gamma radiation, which has a wavelength of 10.sup.-10 to 10.sup.-12 meter and is thus significantly more energetic than ultraviolet radiation having a wavelength of 10.sup.-6 to 10.sup.-7 meter.
It is also known to sterilize polyolefin articles with 2.5 to 5.0 megarads of high energy gamma radiation. However, polymeric articles exposed to such radiation typically suffer from discoloration and embrittlement, which can render them unfit for their intended use.
U.S. Pat. No. 5,371,124 provides a good summary of the various additives which have been proposed to enhance the radiation resistance of propylene polymer compositions prior to molding or shaping the polymer into a useful article. See also U.S. Pat. No. 4,888,369. However, any additive must be compatible with the other components of the polymer composition, and may cause other problems, including objectionable odor and/or color, processing difficulties, bleeding of the additive from the article over time, etc. See, for example, U.S. Pat. No. 4,710,524, which suggests that the inclusion of a mobilizing additive as described in U.S. Pat. Nos. 4,110,185 and 4,274,932 produces undesirable handling and imprinting problems.
Syringe grade material made from polypropylene is typically peroxide visbroken from a polymer having a low melt flow rate (MFR) to obtain a narrower molecular weight distribution and contains oil as a mobilizer to improve the free radical scavenging ability of a hindered amine light stabilizer additive. The polypropylene material typically also contains a sorbitol-based additive as a nucleator.
U.S. Pat. No. 4,876,300 discloses that long chain N,N-dialkylhydroxylamines can be used as process stabilizers for polyolefin compositions in order to minimize discoloration and melt flow rate increase due to extrusion. However, there is no recognition that such long chain N,N-dialkylhydroxylamines can also improve the ductility of a polyolefin composition which has been subjected to high energy gamma radiation.
U.S. Pat. No. 4,668,721 broadly discloses that hydroxylamine derivatives may be used to stabilize polyolefin compositions against degradation due to extrusion, exposure to the combustion products of natural gas, gamma radiation or upon storage for extended periods. The hydroxylamine derivative may have a chemical structure which conforms to one of fourteen structural formulae. Although data is presented for processing stability, resistance to gas fading and resistance to discoloration due to oven aging, no data is presented concerning gamma irradiation. Moreover, almost all of the data is directed to polyolefin compositions stabilized with N,N-dibenzylhydroxylamine; the only dialkylhydroxylamine evaluated was N,N-di-n-octylhydroxylamine.
Recently, European Patent Publication No. 664,315 proposed the use of mobilizing oil and a polysiloxane ether of a hindered piperidine to stabilize polypropylene against sterilizing radiation. There is no disclosure regarding the addition of a N,N-dialkylhydroxylamine to improve ductility of the propylene composition.
An object of the present invention is to provide a ductile polymer composition which can be easily injection molded into flexible, substantially transparent articles.
Another object of the present invention is to provide a gamma radiation resistant polymer composition which is particularly resistant to yellowing.